5 Things We Can Learn From Sounds We Can't Hear

Infrasound is the span of low-frequency sounds below 20 Hz that fall below the hearing range of humans. While these sounds escape our ears, scientific instruments can detect them—and tell us some interesting things about the planet.

Impending Eruptions

Impending Eruptions

Small earthquakes often precede volcanic eruptions. When they do so in rapid succession, they create what's called a harmonic tremor, or a sustained release of infrasonic sound caused by the movement of magma under intense pressure. Some recent examples of volcanoes that scientists know to have produced such a harmonic before erupting include Guatemala's Fuego Volcano, Costa Rica's Arenal Volcano, and Alaska's Redoubt Volcano.

Scientists can measure this sound with a tool called an electret condenser microphone, a type of capacitor microphone designed to eliminate background noise. A series of microphones placed underground in an array detect and record the volcano's infrasonic emissions, then transmit them through underground cables for processing. Together with the volcano's seismic data, this information can help scientists to better understand the activity that occurs within a volcano in the critical moments before an eruption.

By analyzing the earthquake sequence at Alaska's Redoubt Volcano in May 2009, University of Washington doctoral student Alicia Hotovec-Ellis noticed that the frequency of its harmonic tremor grew gradually higher before eruption, a realization she believes may help scientists better understand what occurs during a volcano's eruption cycle. She created two recordings of the seismic activity: the first covering about 10 minutes of harmonic tremor sped up 60 times, and a second highlighting the sounds emitted by more than 1600 earthquakes over an hour.

Nuclear Tests

Nuclear Tests

In 2001, the Preparatory Commission for the Comprehensive Nuclear-Test Ban Treaty Organization (CTBTO) began establishing infrasound stations to measure the low-frequency sound waves that nuclear weapons disperse over long distance in an attempt to expose covert nuclear testing. Today, CTBTO's International Monitoring System (IMS) boasts 60 stations using microbarometers, a sensitive tool that can measure fast fluctuations of atmospheric pressure, to record infrasound. Once these low-frequency sound waves reach an infrasound station, the microbarometer transforms them into electrical impulses, which are then sent to a central recording facility via radio waves or fiberoptics. Computers record the signals and prepare them for data transmission. They're then transferred by satellite to a central facility in Vienna, from where their origins are determined. Using infrasound techonology, the IMS was actually able to determine the location and size of North Korea's first successful nuclear test in 2009.